Give a reason for the following :

(a) Silver nitrate solution is kept in coloured bottles.

(b) Molybdenum is used in the manufacture of ammonia.

(c) A blue solution of copper sulphate changes to green when a piece of iron is added to this solution.

(d) Colourless concentrated sulphuric acid in a test tube changes to blue on adding a small piece of copper to it.

(a) Silver nitrate solution is kept in brown bottles in the laboratory because it decomposes in the presence of light. It becomes black and forms silver, nitrogen dioxide and oxygen.

2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ ↑ + O₂ ↑

(b) Molybdenum acts as a promoter and increases the efficiency of the catalyst iron, hence it is used in the manufacture of ammonia.

(c) When a piece of iron is added to a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green due to the formation of ferrous sulphate.

$\underset{\text{ iron}}{\text{Fe}} + \underset{\text{ blue sol.}}{\text{CuSO}$4}\text{ (aq.)} \longrightarrow \underset{\text{ green sol.}}{\text{FeSO}4} + \underset{\text{ copper-brown red deposit}}{\text{Cu}}

(d) When a small piece of copper is added to concentrated sulphuric acid, a blue solution of copper sulphate is formed.

$\underset{\text{ brown}}{\text{Cu (s)}} + \underset{\text{ conc.}}{2\text{H}$2\text{SO}4\text{(aq)}} \longrightarrow \underset{\text{ blue}}{\text{CuSO}4\text{(aq)}} + 2\text{H}2\text{O (l)} + \text{SO}_2\text{(g)}